Oil field operators demand access to a great quantity of information regarding the parameters and conditions encountered downhole. Such information includes characteristics of the earth formations traversed by the borehole and/or data relating to the size and configuration of the borehole itself. The measured parameters are usually recorded and displayed in the form of a log, i.e., a graph showing the measured parameter as a function of tool position or depth. The collection of information relating to conditions downhole is commonly referred to as “logging”.
Many types of downhole tools exist. One available type of downhole tool is a nuclear magnetic resonance (NMR) logging tool. NMR tools operate by using an imposed static magnetic field, B0, to preferentially align certain nuclei (e.g. Hydrogen) and thereby produce a bulk magnetization. A second magnetic field, which varies in time, is also applied. This field is typically designated as B1 and is traditionally called the “radio frequency (RF) pulse”. After a change in the bulk magnetization due to the radio frequency pulse, the nuclei converge upon their equilibrium alignment with a characteristic exponential relaxation time constant known as the “spin-lattice” or “longitudinal” relaxation time T1. Another relaxation time constant that can be measured is the “spin-spin” or “transverse” relaxation time T2. The tool applies an RF electromagnetic pulse whose magnetic component, B1, is perpendicular to the static field B0. This pulse tips the nuclei's magnetic orientation into the transverse (perpendicular) plane and, once the pulse ends, causes them to precess (“spin”) in the transverse plane as they realign themselves with the static field. The T2 relaxation time constant represents how quickly the transverse plane magnetization disperses through de-phasing and magnitude loss. The precessing nuclei generate a detectable radio frequency signal that can be used to measure statistical distributions of T1 and T2, from which other formation properties such as porosity, permeability, and hydrocarbon saturation can be determined. To enhance the measurement accuracy of the relaxation times, the tool can provide a sequence of radio frequency pulses (such as the well-known Carr-Purcell-Meiboom-Gill “CPMG” pulse sequence) to invert the spin phase and cause the dispersed transverse plane magnetization to gradually refocus into phase, thereby inducing a series of “spin echo” signals.
Measuring relaxation times in these applications requires application of high voltage RF pulses applied to the NMR tool. For instance, a CPMG pulse sequence consists of one excitation pulse and a train of pulses that are separated by predetermined time, called inter-echo time (TE). The applied RF excitation voltages across antenna terminals within the NMR logging tool are often of the order of kV (e.g., 1000-2000 volts). The NMR logging tool is listening for the responsive spin echo that appears between the pulses is of the order of hundreds of nano-Volts. Extremely high amounts of excitation voltage are thus needed to detect relatively insignificant amount of echo response. Smaller inter-echo time (TE) allows for more applied pulses, and the NMR tool can record the spin echo signal as often as possible before the spins are relaxed back to thermal equilibrium.
A limiting factor on the inter-echo time (TE) is known as “ringing”, which capacitors, typically ceramic capacitors, inside the logging tool physically vibrate in response to the applied RF pulses. “Ring-down time” refers to the amount of time it takes to damp ringing from electrical and mechanical energies down to the level of spin echo signal after an RF pulse. The time between pulse and echo must be longer than the ring down time to allow the ringing to dissipate before meaningful echo detection.